fix miri_promise_symbolic_alignment for huge alignments

Author: RalfJung

src/tools/miri/src/shims/foreign_items.rs

@@ -558,14 +558,23 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
 
             // Promises that a pointer has a given symbolic alignment.
             "miri_promise_symbolic_alignment" => {
+                use rustc_target::abi::AlignFromBytesError;
+
                 let [ptr, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
                 let ptr = this.read_pointer(ptr)?;
                 let align = this.read_target_usize(align)?;
-                let Ok(align) = Align::from_bytes(align) else {
+                if !align.is_power_of_two() {
                     throw_unsup_format!(
-                        "`miri_promise_symbolic_alignment`: alignment must be a power of 2"
+                        "`miri_promise_symbolic_alignment`: alignment must be a power of 2, got {align}"
                     );
-                };
+                }
+                let align = Align::from_bytes(align).unwrap_or_else(|err| {
+                    match err {
+                        AlignFromBytesError::NotPowerOfTwo(_) => unreachable!(),
+                        // When the alignment is a power of 2 but too big, clamp it to MAX.
+                        AlignFromBytesError::TooLarge(_) => Align::MAX,
+                    }
+                });
                 let (_, addr) = ptr.into_parts(); // we know the offset is absolute
                 // Cannot panic since `align` is a power of 2 and hence non-zero.
                 if addr.bytes().checked_rem(align.bytes()).unwrap() != 0 {

src/tools/miri/tests/fail/unaligned_pointers/promise_alignment_zero.rs

@@ -0,0 +1,8 @@
+#[path = "../../utils/mod.rs"]
+mod utils;
+
+fn main() {
+    let buffer = [0u32; 128];
+    unsafe { utils::miri_promise_symbolic_alignment(buffer.as_ptr().cast(), 0) };
+    //~^ERROR: alignment must be a power of 2
+}

src/tools/miri/tests/fail/unaligned_pointers/promise_alignment_zero.stderr

@@ -0,0 +1,14 @@
+error: unsupported operation: `miri_promise_symbolic_alignment`: alignment must be a power of 2, got 0
+  --> $DIR/promise_alignment_zero.rs:LL:CC
+   |
+LL |     unsafe { utils::miri_promise_symbolic_alignment(buffer.as_ptr().cast(), 0) };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `miri_promise_symbolic_alignment`: alignment must be a power of 2, got 0
+   |
+   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/promise_alignment_zero.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

src/tools/miri/tests/pass/align_offset_symbolic.rs

@@ -90,9 +90,21 @@ fn test_cstr() {
     std::ffi::CStr::from_bytes_with_nul(b"this is a test that is longer than 16 bytes\0").unwrap();
 }
 
+fn huge_align() {
+    #[cfg(target_pointer_width = "64")]
+    const SIZE: usize = 1 << 47;
+    #[cfg(target_pointer_width = "32")]
+    const SIZE: usize = 1 << 30;
+    #[cfg(target_pointer_width = "16")]
+    const SIZE: usize = 1 << 13;
+    struct HugeSize([u8; SIZE - 1]);
+    let _ = std::ptr::invalid::<HugeSize>(SIZE).align_offset(SIZE);
+}
+
 fn main() {
     test_align_to();
     test_from_utf8();
     test_u64_array();
     test_cstr();
+    huge_align();
 }